In the automobile industry, steel sheets plated with zinc (hereinafter referred to as “galvanized steel sheets”) are widely used because of excellent corrosion resistance, high specific strength, and low cost thereof. In such a case, some types of application require two galvanized steel sheets to be overlaid and welded together. It is said that laser beam welding is preferable, which has excellent characteristics such as the capability of high-accuracy, high-quality, and high-speed processing compared to spot welding and the like.
When two galvanized steel sheets are overlaid and welded with a laser (hereinafter, such welding is also referred to as “laser lap welding”), for example, the galvanized steel sheets are overlaid on top of each other with the galvanized layers facing each other, and irradiated with a laser beam from a carbonic acid gas laser or a YAG laser. Thus, the upper and lower galvanized steel sheets are melted and joined together.
To perform favorable bonding, iron layers of the upper and lower galvanized steel sheets need to interpenetrate. However, the melting point and boiling point of zinc are approximately 420° C. and 907° C., respectively, and there are much lower than the melting point of iron, which is approximately 1535° C. Accordingly, merely overlaying galvanized steel sheets such that galvanized layers face each other and irradiating a weld portion with a laser in that state results in the formation of hole defects (a kind of weld defects) such as pits, porosities, and worm holes due to a phenomenon in which zinc in the galvanized layers blows away surrounding molten metal when evaporating or remains in the molten metal as bubbles.
One countermeasure is to provide a gap of approximately 0.1 mm for venting zinc vapor between galvanized steel sheets to be welded by laser lap welding, and to perform laser lap welding in that state using, for example, a YAG laser having a power of approximately 4 kW at a speed of approximately 3 to 4 m/min.
Furthermore, to efficiently form the gap, the following has been proposed: the vicinity of a portion of one of the galvanized steel sheets to be welded by laser lap welding is irradiated with a laser in advance to be bent, and then, laser lap welding is performed at, for example, a power of approximately 6 kW and a speed of approximately 5 m/min (claim 1 and paragraph 0026 of Patent Document 1).
In addition, for the case in which laser lap welding is performed on three or more galvanized steel sheets, the following has been proposed: for example, in a state in which gaps of approximately 10% of the sheet thickness are provided between the galvanized steel sheets, respectively, welding is performed at a speed of approximately 1.5 m/min by applying a continuous wave having a power of approximately 2.5 kW. This is also based on an idea similar to the aforementioned one to the effect that a gap for venting zinc vapor is formed between steel sheets to be welded (claims 1 and 2 and paragraphs 0019 and 0021 of Patent Document 2).    Patent Document 1: JP-A 2005-144504    Patent Document 2: JP-A 2005-262226